Fix two Clang static analyzer issues

Overflowing memory allocations should crash or exit anyway.

src/common.c

@@ -1072,6 +1072,8 @@ poller_ensure_space (struct poller *self)
 		return;
 
 	self->alloc <<= 1;
+	hard_assert (self->alloc != 0);
+
 	self->revents = xreallocarray
 		(self->revents, sizeof *self->revents, self->alloc);
 	self->info = xreallocarray